Metallized paper is used for decorative paper such as for gift wrappings, and for product identification purposes such as for beer labels, canned food labels and the like. Metallized paper is found to be desirable for such uses because of its glossy aluminized appearance and its related ability to attract the attention of a consumer. Metallized paper is usually printed with some sort of product identifier or some type of decorative figure and is made in varying degrees of gloss level and with various different performance characteristics. For example, gift wrap paper must be easily printable, it must be able to be folded without losing the metal coating, and it must usually have a high reflective finish. Beer labels, on the other hand, must be caustic removable to facilitate their removal during glass reclamation, it must hold up well in a wet environment, and it must also be quite abrasion resistant.
Most metallized paper is made by applying prepolymer and aluminum layers on clay-coated Kraft paper which is approximately 30 to 150 microns thick. The process usually involves applying one or two layers of solvent based prepolymer material and drying them in an oven to remove the solvent after each layer. This method provides a relatively smooth base coating on which an aluminum layer is deposited. The method of first coating the Kraft paper with prepolymer before depositing the aluminum layer is needed because the clay-coated paper is typically not smooth enough to achieve a shiny metallized finish without the smoothing prepolymer layers. After the prepolymer layers are cured, the aluminum is then applied in a vacuum metallizer. A solvent-based prepolymer top coating is applied to the aluminum layer and the solvent is evaporated in an oven. This solvent-based coating process involves at least three or four different steps, increasing the process cost and opportunity for manufacturing losses. Additionally, a very high gloss level cannot be obtained via the solvent-based coating process because of the handling and the solvent evaporation that creates a high density of pinholes in the coating surface, thereby providing a metallized paper having only a medium gloss level. Finally, the use of a solvent-based process is neither environmentally desirable, due to the release of volatile solvent vapors into the atmosphere, nor energy efficient, due to the use of an oven to evaporate the solvent after each layer.
An alternative process to metallize paper on a much more limited basis involves applying an initial smoothing prepolymer layer by using a gravure coating method and curing the layers with a high voltage (150-300 KV) electron beam. The substrate paper is then metallized with a layer of aluminum. A top coat of prepolymer material is applied to the aluminum layer using the gravure method and is cured again using a high voltage electron beam. High voltage electron beams are used because the electron beams are generated inside of a sealed system and they must have enough accelerating voltage to enable them to penetrate through a foil window, through an air layer, and through the coating process. The prepolymer materials that are used in such alternative process are acrylate blends of monomers and oligomers.
The gravure coated acrylate/high voltage electron beam process is more environmentally desirable and energy efficient than the solvent-based coating. Additionally, the gravure process results in a metallized paper coating having improved surface gloss over the solvent-based coating level. The coating is quite sensitive to the wetting of the substrate and the inclusion of bubbles in the coating, ultimately resulting in the formation of pinholes in the surface of the coating. Although the pinhole density associated with the gravure coated process is less than that of the solvent-based process, the ability to obtain a high gloss surface finish is still adversely affected.
The gravure coating process also requires three different process steps and the use of a high voltage electron beam to cure the polymer layer. The use of such high-voltage electron beam not only penetrates the coating layer but penetrates the paper and embrittles it, increasing the probability that the substrate will tear when folded. This curing system is also inefficient because it deposits most of the electron beam energy in the substrate and not in the coating.
It is, therefore, desirable that a metallized paper product and, method for producing the same, be developed that displays a high gloss level without pinholes by using a process having a minimum number of steps. It is desirable that the metallized paper experience no embrittling during the curing process. It is desirable that the coating have excellent adhesion to the paper, have excellent inter-layer adhesion between the prepolymer layers and excellent adhesion between the polymer layer and the metal layer. It is desirable that the method of making the metallized paper be capable of being tailored to particular application requirements for the metallized paper, e.g., to accommodate the creation of a multilayer coating tailored to achieve certain objectives. It is also desirable that the metallized paper be manufactured in a manner that is economically efficient and from materials that are readily available.